1. Field of the Invention
The present invention relates to a heating arrangement and more particularly relates to a heating arrangement suitable for use in heating the seat in a motor vehicle.
2. Description of the Related Art
It has been proposed previously to provide a heater integrated into the seat of a motor vehicle in order to heat the seat. Of course, it is necessary to provide some type of thermostatic control for such a heater.
It has been proposed to utilise an electromechanical relay, to effect such thermostatic control. However, such a relay, because it operates in a mechanical manner, cannot make a very large number of different "makes" and "breaks". Consequently such a relay must be adapted to operate with a relatively large hysteresis loop, for example, making at 15.degree. C. and breaking at 35.degree. C. Whilst such a relay may comprise a switch operated by relay coils in response to signals from a temperature sensing arrangement, another example of this type of arrangement is a bi-metallic thermostat switch, in which both the temperature sensor function and the switching function are embodied in the same device. Such a device does not need a control circuit, and is therefore very cheap. However, the big hysteresis loop that is necessary proves to be a disadvantage.
Other types of temperature sensors have been proposed for use in such an application, for example a thermistor, which is a resistor having a negative temperature dependence, or the base-emitter voltage of a transistor. However, if temperature sensors of this type are utilised a control circuit is needed to actuate the relay. This requires a control box or circuit which is located outside the seat squab, while the sensor is placed in the seat squab.
It would, of course, be possible to utilise a heating wire which has a positive, but very small, temperature dependence as the temperature sensor. In such an arrangement the resistance of the heating wire is monitored to provide the necessary control. However, this requires the provision of a complicated, and thus expensive, control circuit.